The invention relates to a method for separating a gas mixture according to a pressure swing adsorption process (PSA process), including the step of separating the gas mixture by adsorbing at least one gas component in an adsorbent mass provided in each vessel of a plurality of vessels, each vessel having at least one inlet and one outlet.
US 2009/0020014 A1 discloses a PSA system having four adsorbent vessels. These adsorbent vessels are connected to four parallel flow manifolds, namely a feed manifold, a product manifold, a waste gas manifold and an equalization and purge manifold. These vessels are provided with corresponding raw gas feed valves and equalization and purge valves. The mechanical arrangement disclosed thus possesses four vessels with four valves each.
A PSA system is used for instance to purify hydrogen gas in the production of pure hydrogen (the product gas) from synthesis gas or reformate (the feed gas). The pressure swing adsorption process is based on the difference in adsorption at high pressure and at low pressure for the different components on the specific absorbent. A high amount of compounds like methane, carbon dioxide, carbon monoxide and nitrogen, whereas only a relatively small amount of hydrogen will be adsorbed.
In a prior art PSA system, in each of the vessels seven steps are to be distinguished, however out of phase:
(i) production, (ii) equalization (pressure reduction), (iii) addition of purge gas, (iv) blow-down, (v) reception of purge gas, (vi) equalization (pressure increase), (vii) final re-pressurization.
On purifying hydrogen gas in a four vessel prior art PSA system, when the first vessel (vessel 1) is in the production step, the gas mixture flows through a cleaned adsorbent bed at high pressure. The compounds other than hydrogen are adsorbed in the adsorbent bed. Part of the produced pure hydrogen is used for the final re-pressurization of the next vessel in production (vessel 2). When the adsorbent bed is partially saturated, the adsorbent bed is disconnected from the feed stream, and vessel 2 takes over production. The product side of vessel 1 contains still pure hydrogen, the feed side contains the contaminated hydrogen.
Vessel 1 is then switched to the equalization phase. In this phase the clean sides of vessel 1 and the third vessel (vessel 3) are connected. Vessel 3 is the vessel that will take over hydrogen production from vessel 2. Vessel 3 has been cleaned, but is still at low pressure. Clean hydrogen will flow from the high pressure vessel 1 to the low pressure vessel 3 until both vessels have (almost) the same pressure. As a result, the hydrogen released during the depressurization of vessel 1 is not lost, but is utilized for the pressurization of vessel 3.
Next, the vessel 1 provides gas to purge the fourth vessel (vessel 4). Vessel 4 is at low pressure, and therefore the adsorbed gasses will desorb in an effort to restore the equilibrium pressure corresponding to the amount of the adsorbed gas adsorbed to the absorbent. Flushing vessel 4 with pure hydrogen from the product side of vessel 1 will further reduce the partial pressure of the adsorbed contaminants. Vessel 4 will be cleaner when this proceeds at a lower pressure.
Next, vessel 1 is going through the blow-down phase. In this phase the pressure is reduced by removing gas from the feed side. In this phase already contaminants are removed from vessel 1. The adsorbed gasses at the adsorbent are in equilibrium at the adsorption pressure. Reducing the pressure will result in desorption of the contaminants in an attempt to keep the absolute partial pressure of the contaminants constant.
After the blow-down phase, vessel 1 is purged using pure hydrogen from vessel 2. Usually this is done by opening valves, and because of the much higher pressure of vessel 2, vessel 1 will experience a pressure hill with a maximum pressure depending on valve sizes and adsorbent bed packing and size somewhere between the pressure of vessel 2 and vessel 1 at the start of this phase. The partial pressure of the contaminants in vessel 1 is now reduced to a very low level due to the low pressure and the low concentration in the purge gas. At the end of the purge the clean-up of vessel 1 is finished.
Now the pressure in vessel 1 has to be increased to the production pressure. The first step of the pressure increase is the equalization. In this step vessel 1 receives pure hydrogen from the product side of vessel 3. Roughly 50% of the required pressure increase is effectuated in this step.
The final step in the cycle is the final re-pressurization. In this phase a part of the hydrogen that is purified by vessel 4 is fed to the product side of vessel 1, until the production pressure is reached. Vessel 1 is now ready to start production again.
It is perceived to be a drawback of purifying hydrogen gas in a prior art PSA system, that the gasses produced in the blow-down and the purge phase still contain a significant amount of hydrogen and possibly methane and carbon monoxide, depending on the hydrogen production method.
Usually these gasses are used in a burner. Often a burner that supplies the heat for the steam reforming reaction or for steam generation or a combination of these. These burners however need a continuous flow of fuel, where the availability of these flows is very discontinuous. This is usually handled by a large off-gas buffer vessel. All rejected gas from the blow-down and purge are gathered in this off-gas vessel. The burner consumes these gasses from the off-gas vessel, which however causes the pressure in the off-gas-vessel to fluctuate. This means that the lowest pressure for the end of the blow-down and for the complete purge is the pressure of the off-gas vessel at that point in the cycle. To limit this effect the off-gas vessel can be designed very big, but that is expensive.
DE 69935838 T2 discloses a pressure swing adsorption (PSA) gas separation process which comprises delivery of adsorber residual gas to different pressure distribution pipes during different pressure stages of adsorber decompression/regeneration. The PSA gas mixture separation process comprises delivering residual gas from an adsorber to: (a) a first residual gas distribution pipe under a first distribution pressure during a first decompression/regeneration stage, in which the adsorber pressure is within a first intermediate pressure range between the high pressure and the low pressure of the cycle; and (b) to a second residual gas distribution pipe under a lower second distribution pressure during a second decompression/regeneration stage, in which the adsorber pressure is within a second pressure range between the low pressure of the first pressure range and the low pressure of the cycle.
According to this prior art process, the residual gas is stored in either a buffer vessel at the first distribution pressure, or in a buffer vessel at the second distribution pressure, and the adsorber pressure has a lower limit which is given by the pressure in the actual buffer vessel.
For a proper efficiency of the process, it is required that in the step (v) described above, during which purge gas is received in the adsorber, the adsorber pressure is very low. Therefore, the pressure in the actual buffer vessel constitutes serious obstacle for the release of residual gas from the adsorber.
JP 2005 289730 A discloses a method and an apparatus for producing hydrogen for producing high purity product hydrogen from a hydrogen-enriched gas by repeating a hydrogen taking-out step for taking out the high purity product hydrogen by adsorbing impurities in the hydrogen-enriched gas onto the adsorbing agent while maintaining the inside of the hydrogen purification unit in a pressurized state. The process comprises an off-gas taking-out step for taking out off-gas by desorbing the impurities from the adsorbing agent while maintaining the inside of the hydrogen purification unit in a reduced pressure state, and a cleaning step for cleaning the adsorbing agent by high purity cleaning hydrogen while maintaining the inside of the hydrogen purification unit in a reduced pressure state, after completion of the hydrogen taking-out step. A hydrogen recovering step for recovering the high purity hydrogen remaining in the hydrogen purification unit as the cleaning hydrogen is performed by the reduction of the pressure in the hydrogen purification unit, and after completion of the hydrogen recovering step, the off-gas taking-out step is performed.
In this prior art process, residual gas is stored in a buffer vessel, and the adsorber pressure has a lower limit which is given by the pressure in this buffer vessel.